The present invention pertains to hydraulic pumps for delivering high-pressure fuel to common rail fuel injection systems for internal combustion engines.
A typical gasoline direct injection (GDI) pump is sized by the maximum fuel demand, which occurs at extremely cold starting conditions. This means that during 99% of pump operation, such a pump is highly oversized. The oversizing produces excess pressurized fuel and the problem arises as to handling the unwanted highly pressurized fuel. This has been one motivating factor for the development of so called “demand controlled” pumps.
With the automotive industry looking to increase common rail pressure to 200 bar or more, the weaknesses of current demand-based fuel control techniques are becoming even more evident. Currently, three mainstream methods of demand control are known:
1. High Pressure Bypass
Pressurized fuel is spilled (either at the pump or from the rail) back into the low pressure circuit (back to the tank or into pump inlet). This method provides very uniform pressure and low pulsation drive torque, but is very inefficient and also poses serious problems because of heat rejection.
Systems like these are successfully used today with pumps delivering up to 0.6 cm3/rev and up to 120 bar pressure, but any further pressure and/or output increase would require an additional fuel cooler in order to keep the temperature of the system components within acceptable levels.
2. Low Pressure Bypass
The pumping chamber is fully filled prior to each pumping event and the unwanted fuel quantity is spilled before high pressure is generated. This method is more efficient then the previous one and also results in far less heat rejection. However, with ever increasing demands for higher output and higher pressure level, the efficiency is likely to suffer and it also will present higher and higher technical challenges, to achieve the desired effect. A high speed, high flow and high force control solenoid is required and this means also a high power driver to control this solenoid will be required.
Another potential drawbacks of this approach is achieving adequate durability despite the very high number of working cycles over the expected vehicle lifetime.
3. Inlet Metering
This is by far the most efficient method, as only the desired amount of fuel is pressurized and because only low pressure fuel is controlled, a low power, slow control solenoid is satisfactory. However, this method has its own serious drawbacks.
(a) Uniformity of operation: At full output the pumping characteristic for a three plunger pump is relatively smooth. However, at part load, until the pumping events start to overlap, there will be three distinct pumping events per revolution. With six or more cylinder engines, the rail pressure for every other injection event will be lower than for the previous one, because the rail was not refilled in between and rail pressure determines ultimately the exact injection fuel quantity. A second issue regarding the pumping uniformity is the case when pre-metered fuel quantity is supplied into the charging circuit (for example by using typical MPFI gasoline injector). As charging conditions of all pumping chambers are not exactly identical (gravity, individual tolerances of orifices and clearances, friction, inlet check spring forces, distance from the solenoid etc.) the fuel quantity supplied by all three pumping events will not be identical. In the worse case at some small quantities, only one pumping event per revolution could take place.
(b) Hydraulic and acoustic noise: Because each pumping chamber is only partially filled prior to the injection, collapsing of vapor cavities will generate audible and hydraulic noise. Although under some circumstances when the pumping rate remains relatively low, this cavitation will not necessarily translate into erosion, the audible noise might pose a serious problem, especially at low speeds, for example at idle, when there are no other noises to mask (cover up) the noise generating by the pump and when the operator might be most sensitive as far as noise is concerned.
(c) Transients: Both ascending and descending transients will be delayed by at least 180 degrees of rotation from intention tot implementation time, because any change in desired output can only be implemented after the charging cycle is completed. This delay will negatively affect the smoothness of engine operation, especially at low speed, where 180 degrees translate into longer time. For example, at 3000 engine rpm the delay time would be about 20 ms, whereas at 200 rpm the delay time would be almost 300 ms. During ascending transients at least three injection events have to pass, before the increased injection quantity-takes place. During descending transient the pump will deliver more fuel than needed, resulting in a rail pressure increase up to the pressure limiter level setting. This will lead to higher than desired injection quantity when the fuel demand resumes. In a typical case, during the gear-shifting event, there is an instantaneous demand for zero fuel, as the driver repositions his foot from throttle to the clutch and back.
(d) Controllability: The inlet metering orifice has to be sized to insure maximum quantity of fuel at the maximum pump speed. Because the time available for charging at low speed is much longer, there will be a very small difference between the pulse width corresponding to wide open throttle (WOT) versus pulse width corresponding to almost zero load, making the control of the exact amount of fuel very difficult. This can be exemplified by the calculated output of a pump rated at 200 bar pressure, with 1000 mm3/rev displacement and 442 mm3/rev WOT, operating with conventional inlet metering via a proportional solenoid control. At 750 rpm the desired WOT fuel is achieved at 1% of the solenoid duty cycle, making control of any smaller fuel quantity, for example 10% WOT, virtually impossible. At 1300 rpm the duty cycle range required to control fuel quantity between zero and WOT, would be a more manageable 0 to 30%.